Frequency shift keying of an audio tone is a well known method of transmitting data. In general, the tone frequency is generated by an LC or RC oscillator which produces a relatively clean sinewave and, if the circuit is designed accordingly, amplitude can be made to remain fairly constant and there will be no phase discontinuities. Also, the time delay between keying the oscillator and the time that the frequency actually shifts can be made extremely short. However, there is a limit to the frequency accuracy of an LC or an RC oscillator, particularly with respect to temperature and time.
A quartz crystal controlled oscillator has a very high frequency accuracy and frequency stability versus age and environmental conditions. Crystals operating in the audio frequency range are bulky and expensive, whereas crystal operating in the megahertz range are inexpensive, readily available, small and highly accurate.
In accordance with this invention, high frequency crystal controlled oscillators are operated continuously, there being one oscillator for each of the FSK frequency states or levels to be generated. These oscillators are gated, one at a time, to the input of a digital frequency divider whose output is the required tone frequency. Since the data input may be asynchronous, and the gating circuits and digital frequency divider are zero crossing devices, there will be a time error at crystal frequency if the frequency is commanded to shift non-coincident with a zero crossing of the oscillator frequency. However, the state of the art now is such that 5-10 megahertz crystal oscillators are practical using C-MOS logic so that the time error is of the order of a few hundred nano seconds, which is insignificant at the final audio tone frequency. This time error refers to the error between zero crossings at the input of the digital frequency divider. The zero crossings at the output, however, will not necessarily occur simultaneously with a command to shift frequency. Therefore, there can be an asynchronous timing error which for some systems may be substantial. The timing error or asynchronous delay can be 1/2 cycle of the divider output frequency. If the FSK detection means at the receiving end of the communications channel is responsive only to zero crossings, then the switched crystal controlled oscillators and the digital frequency divider will not increase the asynchronous timing error above that already generated by the said zero crossing detection means. Theoretically, it is possible by use of a continuous phase FSK demodulator at the receiver to have no asynchronous timing error. However, the output of such a perfect demodulator changes when the rate of change of phase of the incoming signal changes. Continuous phase information over the entire tone frequency cycle is not available from zero crossings alone. Tone generation, transmission and detection of a sinewave is required. Because the crystal frequencies are digitally divided down to tone frequency, a squarewave is generated. The use of a continuous phase discriminator at the receiving end would not improve the asynchronous timing error.
The use of switched crystal controlled oscillators with subsequent frequency division down to audio tone frequency is not new. The invention is directed to the solution of problems inherent in such systems. Specifically, a continuous phase signal is generated by processing the output of the digital frequency divider through an active tracking bandpass filter. The rate of change of phase of the output signal changes virtually instantaneously when the circuit is shifted from one oscillator to another. Therefore, the combination of digital tone generation and tracking active bandpass filter produces no asynchronous timing error and the signal is suitable for reception at the receiving end of the system by either continuous phase FSK detection means or zero crossing FSK detection means.